lavf/flacdec: Share the function to parse cover art blocks

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vorbiscomment: Add support for embedded cover art

Fixes ticket #2655

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>